Wind turbines are located in areas that are unprotected from meteorological conditions in order to tap the full potential of the wind.
Due to this siting in exposed areas and due to the shape of wind turbines, there is a high probability of lightning strokes which may cause severe damage. For this reason, wind turbines are equipped with lightning protection systems for protecting their components against lightning impacts.
As shown in the simplified schematical figure FIG. 3, wind turbines, generally, comprise a tower 23d, a nacelle and a rotor system 83.
The bed frame 103 of the nacelle is connected to the tower 23d via a yaw system for enabling movement of the nacelle on top of the tower 23d. The bed frame 103 further supports the drive train of the wind turbine which is installed inside of the nacelle.
The rotor system 83, comprising hub and rotor blades, is connected to the drive train. The drive train itself comprises at least a shaft 93, a generator 113 and a braking system 13c, 23c and may further comprise a gearbox 123. The braking system comprises a brake calliper 13c and a brake disk 23c. 
In case of a lightning stroke, significant parts of the lightning current may pass through or near all wind turbine components. Thus, the lightning current has to be directed over an electrically conducting path to ground in such a way that damages to the components are avoided.
Some of the components on this conducting path have to be electrically coupled although they are moveable in relation to each other.
Those interconnections are, beside the hub/nacelle interconnection, also the yaw clamp/tower interconnection 13d,23d on top of the tower and brake calliper/brake disk interconnections 13c,23c, wherein the brake calliper/brake disk interconnections 13c,23c may be located on the generator shaft or elsewhere in the wind turbine.
In this context, FIG. 4a and FIG. 4b show a prior art realisation of a yaw system comprising a lightning current protection unit for transferring a lightning current from the nacelle 14 to the tower 24 of the wind turbine.
FIG. 4a shows a cross-sectional view and FIG. 4b shows a topview of this realisation.
The realisation comprises basically a block-shaped conductive brush 44, a spring 54 for pressing the brush 44 onto the yaw ring, a wire 144 for connecting electrically the brush 44 to the nacelle 14 and a bracket-shaped lightning brush casing 134 in which the brush 44 and the spring 54 are located.
The parts of the unit are assembled in advance, and then, the entire assembly is mounted with bolts 154 between the yaw clamps 14a, 14b. 